The invention relates to a sensor and an operating method of the sensor for the detection of soot in a measurement gas volume. Both the establishment of whether soot is present in the measurement gas and the concentration thereof are considered.
The accumulation of carbon dioxide in the atmosphere is being discussed from a variety of standpoints nowadays. This is associated with the fact that the availability of fossil fuels is limited. As a reaction to this, by way of example, combustion processes are being optimized thermodynamically, such that their efficiency is improved. In the motor vehicle sector, this is manifested in the increasing use of diesel vehicles. The disadvantage of this combustion technology however, in comparison with optimized spark ignition engines, is a significantly increased emission of soot. In addition, the formation thereof virtually cannot be prevented by combustion-technological measures. Soot is highly carcinogenic particularly due to the deposition of polycyclic aromatic compounds (PAH), and various regulations have already reacted to this. Thus, for example European exhaust gas emission standards are associated with maximum limits for soot emission. Therefore, there is the need to specify an inexpensive sensor technology that measures the soot content in exhaust gases.
The use of such soot sensors can be used both for the measurement of the soot currently being emitted, in order that the engine management in an automobile acquires information in a current driving situation in order to reduce emissions by means of technical control adaptations, and for performing active exhaust gas purification by means of so-called exhaust gas soot filters. In the latter case, use is made of regenerable filters which filter out a substantial part of the soot content from the exhaust gas. Soot sensors are required for the detection of soot in order to monitor the function of the soot filters, or in order to control the regeneration cycles thereof.
There have been various approaches for detecting soot in the prior art. An approach that has been pursued to a great extent in laboratories consists in using the light scattering by the soot particles. This procedure is suitable for complex measuring instruments. Where an attempt is made to use this also as a mobile sensor system in exhaust gas, it must be established that such approaches for realizing a cost-effective sensor in a motor vehicle are associated with high costs owing to the complex optical construction. Furthermore, there are unresolved problems with regard to the contamination of the required optical windows by combustion exhaust gases.
The published German patent application DE 199 59 871 A1 describes a sensor and operating method, both of which are based on thermal considerations. The sensor comprises an openly porous shaped body such as, for example, a honeycomb-shaped ceramic, a heating element and a temperature sensor. If the sensor is brought into contact with a measurement gas volume, then soot deposits on it. For measurement purposes, the soot deposited over a period of time is ignited with the aid of the heating element and burned. The temperature increase that arises during combustion is measured. Although this is a practicable procedure in constant environmental conditions, under the conditions of a motor vehicle exhaust section with greatly fluctuating flows and exhaust gas temperatures the measurement of the relatively small temperature increase proves to be an extremely difficult problem.
In principle, it is possible to use two methods for measuring soot. One is based on an electric field between two electrodes at which the measurement gas volume loaded with soot is present, the soot causing an ionization current. One embodiment of this principle is known from the published German patent application DE 102 44 702 A. In this case, the exhaust gas flow passes two electrodes which are provided with an electrical insulation layer and between which is situated the soot-containing gas to be examined.
The electrodes are operated with an AC voltage of between 1 and 10 kV. Between the electrodes, depending on the soot concentration in the measurement gas, a dielectrically impeded discharge is performed and the currents that occur are measured. For motor vehicle operation, this method is not practicable insofar as its realization requires high voltages and complicated measurement technology. Secondly, the changing gaseous exhaust gas constituents will entail a significant measurement corruption as a result of interfering gas influencing the ionization current.